1. Field of the Invention
This invention relates to video assist systems for motion-picture cameras. More specifically, this invention relates to a television video assist system for converting motion-picture camera images to television signals suitable for broadcast, display or recording.
2. Description of Related Art
Some advantages which may be obtained from video assist systems for motion-picture cameras are disclosed in U.S. Pat. No. 4,928,171, "VIDEO ASSIST SYSTEM FOR MOTION-PICTURE CAMERA", hereby incorporated by reference as if fully set forth herein. As used herein, a "video assist system" is a system which duplicates the image available to the motion-picture camera operator (the "operator") for presentation at another location. Typically a television ("TV") camera will be used. One of the primary problems which is encountered in the art is flicker, which may be caused by mismatch between the frame capture rates of the motion-picture camera and the TV camera. As used herein, a "video flicker processor" is a system which operates to remove objectionable flicker.
One problem in the art is that optics which are used with video assist systems in motion-picture cameras typically require that light must be shared between the operator and the TV camera. Thus, a sensitive TV camera may be needed. This problem is particularly acute with color TV cameras, which may be as little as one-tenth as sensitive as black and white TV cameras.
Another problem in the art is that due to the arrangement of optics, the TV camera may record a mirror image of the actual scene. This may require either an additional mirror in the optical system, or may require additional processing by the video assist system.
Some well-known differences between motion-picture and TV cameras are as follows. A motion-picture camera typically captures 24 frames per second, but may have a frame capture rate which is set by the operator. Each frame thus comprises a shutter-open portion, typically lasting 1/48 of a second, and a shutter-closed portion, also typically lasting 1/48 of a second. In contrast, a TV camera typically captures 30 frames per second (in NTSC format, as is well known in the art), each comprising 525 horizontal lines. Each frame is displayed as two consecutive fields, each comprising 262.5 horizontal lines and lasting 1/60 of a second. Half of the fields comprise even-numbered lines of the frame ("even fields"), while half comprise odd-numbered lines ("odd fields"); even fields and odd fields are interlaced upon display.
One method is to use a TV camera in a video assist system which is synchronized with the motion-picture camera so that one field of the TV signal (lasting 1/60 second) is transmitted for each motion-picture frame (lasting 1/48 second). Generally, only one kind (i.e., even or odd) of field is transmitted by the TV camera. (During the shutter-closed portion, the TV signal is ignored.) When the one field of the TV signal is transmitted at a standard TV-format rate, there remains a substantial time interval for the shutter-open portion of each motion-picture frame during which no video signal is transmitted. The TV signal has less brightness because only one field is used.
One problem with this method is that the TV camera must be phase-synchronized with the motion-picture camera so that each frame which the TV camera captures comprises an equal amount of light, i.e., each frame has an equal proportion of time during which the motion-picture camera shutter is open. If the motion-picture and the TV camera are not synchronized, successive frames may vary in overall brightness and the image will flicker.
One method for converting the motion-picture image to a TV image is similar to methods used for converting between different TV-format standards, such as between NTSC and PAL. (Both of these TV formats are well-known in the art.) These methods for converting standards may use a video buffer (also called a "frame grabber") which will typically store over 1 megabit of memory with access times of under 100 nanoseconds ("nsec") per bit.
One problem with this method is that control and timing of a typical video buffer may be complex and therefore subject to error. For example, color and synchronization ("sync") signals must be removed from the video signal prior to storage in the video buffer, and must then be reinserted into the video signal when it is retrieved.
Another problem with this method is that mismatch between the frame capture rates or frame display rates of the motion-picture camera and the TV camera may cause jerkiness or other motion artifacts in the displayed image. For example, when the motion-picture camera captures 24 frames per second, and the TV camera captures 30 frames per second, some of the motion-picture images (or some parts of those images) must be repeated to generate a smooth TV signal.
A particular type of motion artifact may be generated when the TV signal comprises interlaced fields which are stored in a field buffer. When a field is repeated (or some part of a field is repeated) from the field buffer, it may be a field which is earlier in time than the previous field, causing a motion artifact which can be noticed by the viewer.